Radio Frequency Identification (RFID) systems represent the next step in automatic identification techniques started by the familiar bar code schemes. Whereas bar code systems require line-of-sight (LOS) contact between a scanner and the bar code being identified, RFID techniques do not require LOS contact. This is a critical distinction because bar code systems often need manual intervention to ensure LOS contact between a bar code label and the bar code scanner. In sharp contrast, RFID systems eliminate the need for manual alignment between an RFID tag and an RFID reader or interrogator, thereby keeping labor costs at a minimum. In addition, bar code labels can become soiled in transit, rendering them unreadable. Because RFID tags are read using RF transmissions instead of optical transmissions, such soiling need not render RFID tags unreadable. Moreover, RFID tags may be written to in write-once or write-many fashions whereas once a bar code label has been printed further modifications are impossible. These advantages of RFID systems have resulted in the rapid growth of this technology despite the higher costs of RFID tags as compared to a printed bar code label.
Generally, in an RFID system, an RFID tag includes a transponder and a tag antenna, which communicates with an RFID transceiver pursuant to the receipt of a signal, such as an interrogation or encoding signal, from the RFID interrogator. The signal causes the RFID transponder to emit via the tag antenna a signal, such as an identification or encoding verification signal, that is received by the RFID interrogator. In passive RFID systems, the RFID tag has no power source of its own and therefore the interrogation signal from the RFID interrogator also provides operating power to the RFID tag.
Currently, a commonly used method for encoding the RFID tags is by way of an inductively coupled antenna comprising a pair of inductors or transmission lines placed in proximity of the RFID transponder to provide operating power and encoding signals to the RFID transponder by way of magnetic coupling. Magnetic coupling, however, is not without shortcomings. Magnetic coupling generally depends on the geometry of the RFID tag, such as the shape of the tag antenna, transponder, etc, so an often complex process for determining an optimal alignment of transceiver with the RFID tag is necessary for effectively directing the magnetic field between the transceiver and the RFID tag such that their magnetic fields would couple. Furthermore, this process has to be redone if the transceiver is be used for encoding an RFID tag of a different geometry, due to a different shape or a different orientation with respect to the pair of inductors when placed in proximity of the RFID transponder.
An attractive alternative to magnetically-coupled RFID encoding schemes are capacitively-coupled RFID encoders. For example, U.S. Ser. No. 11/073,042 (the '042 application) filed Mar. 4, 2005 describes a capacitively-coupled RFID encoder. Unlike conventional near-field capactively-coupled encoders, the encoder described in the '042 application requires no modification to the encoded tag. In contrast, conventional near-field techniques typically require the RFID tag antenna to be modified with capacitive plates. However, the '042 application describes an electromagnetic modeling technique to determine areas of relatively high current when a conventional RFID antenna such as a dipole antenna is excited by RF energy.
The encoder disclosed in the '042 application exploits these areas of relatively high current by providing matching capacitive elements in the encoder. These capacitive elements are selected to be proximate the high current areas. Thus, when the capacitive elements are excited by an RF encoding signal, the adjacent RFID tag antenna will respond to this capacitive excitation.
Despite the advances disclosed in the '042 application, there remain unfulfilled needs in the art. For example, a user of a capacitive encoder often desires to know whether the RFID tag being capacitively encoded is operative. Accordingly, there is a need in the art for an improved capacitively-coupled RFID transponder test system.